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AECbytes Tips and Tricks Issue #27 (February 19, 2008)

Processing Image Stacks in Photoshop CS3 Extended

Scott Onstott
Book & Video Author

Everyone knows that a talented person can work wonders on an image using Photoshop. Some techniques require more skills and experience than others, and those techniques with the steepest learning curves are often left to visualization specialists. For a change, this month I'd like to show you a technique that is so easy everyone can do it—one that gives a tremendous bang for the buck.

Photoshop CS3 Extended has a new feature called image stacks that gives you the ability to place and align related images within a smart object. A new kind of processing can be done across all the images in the stack—yielding some interesting and useful results. For example, see the images shown below—all twenty images were shot of the same space, with the photographer standing in the same position. The photographer just kept pressing the shutter as people walked through the public space. Notice that every single picture contains people. It might not be obvious from the thumbnails, but every photo's perspective is slightly different as the photographer handheld the camera and breathed in between shots. You can download these shots here.

Step 1: Load images into stack

Combining these images into an image stack is done through a script that ships with the Extended version of Photoshop. Choose File > Scripts > Load Files into Stack. Choose either Files or Folder in the drop-down menu and then click the Browse button. Select the individual photos or their containing folder. If you go the folder route, make sure there are no other, non-related photos in the folder you select, or they too will get combined into the stack.

Check Create Smart Object after Loading Layers and Attempt to Automatically Align Source Images and click OK. It will take a few minutes for Photoshop to create the image stack. If you used a tripod, then you can leave Attempt to Align Source Images unchecked. It sure is convenient that Photoshop can now analyze similar images and line them up automatically so you can leave your bulky tripod back at the studio.

Step 2: Crop and adjust stack

Once Photoshop has finished loading and aligning images in a stack, you'll get a single smart object layer—automatically taking its name from the top photo in the stack. In this case, my layer was called IMG_080201.jpg. I suggest renaming this layer Image stack for clarity. Double click the Image stack layer thumbnail to open the smart object. Inside you'll find 20 layers corresponding to the individual photos that were loaded into the stack. There's nothing you have to do inside the stack, just open it to understand how it's structured. Press Command-W (PC: Ctrl+W) to close the image stack document and return to the main project window.

I feel that the images in this stack are all a bit dark. You can adjust the exposure throughout the entire stack by applying filters, adjustments or adjustment layers to the smart object. In this case, choose Image > Adjustments > Shadow/Highlight. Increase the shadow amount and tonal width so that the shadows get a little brighter and click OK. Shadows/Highlights appears as a smart filter under the Image stack layer, meaning it remains adjustable.

The photos were auto-aligned because they were shot without a tripod. Photoshop shuffled them around ever-so-slightly but now the outer edges of all the photos don't quite line up. The solution: crop the ragged edges away using the Crop tool. Press C, drag out a rectangle, and adjust it with the handles. Press Return (PC: Enter) to crop.

Step 3: Apply stack modes

Now for the amazing part. Choose Layer > Smart Objects > Stack Mode > Median. After a short processing delay, you'll get a clean architectural shot, meaning that all the people are gone. Median analyzes all the variations that take place across the stack and returns pixels that are present more than 50% of the time. It actually removes noise as well! Median can remove tourists, planes, trains, automobiles, or anything that moves. It won't get rid of people sitting on a bench or parked cars, but it can greatly reduce the amount of Clone Stamp and/or Healing Brush work you need to do to get a clean architectural shot.

There are ten other stack modes to play with. Median is probably the most useful, but there are a couple of others worth mentioning. Maximum reveals the specular highlights from all the images in the stack, as shown below. You can use Maximum stack mode to study pedestrian flow patterns through the plaza. People appear as specular ghosts that float through the space—it's not exactly scientific data but it does give you a good qualitative sense. Range is one of the many abstract stack modes that are available. Range is almost like an artistic filter, but its processing takes place across all the images in the stack, so it's really a different animal.

As you can see, stack modes are a very interesting development in digital imaging, and very easy to use.

About the Author

Scott Onstott is a book and video author of AEC software tutorials. He has a degree in architecture from UC Berkeley and has served as an instructor there, in addition to working in several prominent engineering, architecture, and interiors firms in San Francisco. He has also worked as a technical editor and technology consultant.

Scott has contributed to over two dozen books and videos on AutoCAD, Architectural Desktop, VIZ Render, Revit, 3ds Max, VIZ, Photoshop, Illustrator, Painter, Fireworks, and Dreamweaver. He can be reached via:

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